This invention relates generally to the field of threaded connectors and in particular to threaded connectors for tubular goods.
The need for a threaded connector for tubular goods having an axial tensile capability substantially equal to that of the tubular goods has long been recognized. By tubular goods it is to be understood that drill pipe, work tubing, production tubing, well casing and other such conduits used in a well are included. It is also to be understood that the use of the threaded connector of the present invention is not to be considered limited to such tubular goods.
Previously, in achieving the tensile strength need in tubular goods certain undesired features resulted. For instance, the American Petroleum Institute (API) Buttress thread connection utilizes vanishing, tapered threads to achieve a loading capability approximating full tensile capacity of the tubular member. However, the high radial thread interference required to resist separation of the connected tubular members under axial loading creates a high surface contact stress that results in galling and other localized thread damage that severely limits the number of times the connection can be made up. In addition to providing only limited usability of the thread connection, the area of high radial interference stress is especially susceptible to a form of stress corrosion cracking that is found to occur in petroleum well conduits known as sulfide stress cracking. For an example of such a threaded connection having negative angled load flanks, see U.S. Pat. No. 3,224,799.
To overcome the problem of radial interference stress in the threaded connection, threaded connections for tubular goods having radial clearance for all but the loading flank of the threads have been developed. U.S. Pat. No. 1,932,427 to Stone discloses such a radial clearance in a two-step tapered thread connection formed on a pipe upset having a greater stabbing depth to prevent cross-threading. U.S. Pat. Nos. 2,006,520 and 2,211,179 disclose additional variations of the non-tapered two-step threads having radial clearance and which provide make-up shoulders for increasing the usability or number of times the threaded connection may be made up. In U.S. Pat. No. 2,992,019, an inner seal that is responsive to the internal fluid pressure is disclosed for use with such radial clearance two-step threads.
U.S. Pat. No. 3,989,284 to Blose discloses a threaded connection in which the pin or male member is placed in hoop tension when the connection is made up due to the dove-tail and wedging engagement of the threads. The formation of this tapered and dovetail thread profile is both complex to manufacture and very expensive.
In Blose U.S. patent application, Ser. No. 679,476, filed Apr. 22, 1976, now abandoned, assigned to the assignee of the present invention, there is disclosed a two-step thread connection having a negative angled "hook-thread" engagement and which has a tensile strength greater than the pipe body. To obtain that strength it was necessary to form the threads on collars having a wall thickness greater than the pipe wall thickness to which the collars were then welded. The rigid thick walled collars for this connection made such connections difficult to install in the well bore as they required significant outer annular clearance.
Each of the foregoing mentioned patents is hereby totally incorporated herein as written disclosure by this specific reference as is fully set out herein.